This invention relates to a cathode-ray tube, such as a color cathode-ray tube, and more particularly to a cathode-ray tube apparatus capable of reducing the deflection power effectively and securing the strength of the vacuum envelope to atmospheric pressure.
A cathode-ray tube, such as a color picture tube, has a vacuum envelope made of glass and composed of a panel with an almost rectangular display section, a funnel connected to the panel, and a neck of cylindrical shape connected to the funnel. A deflecting yoke is provided between the neck and the funnel. The funnel has a decreased-diameter section, or a yoke section, ranging from the junction with the neck to the position where the deflecting yoke is provided.
On the inside face of the effective portion of the panel, a fluorescent screen composed of a three-color fluorescent layer of blue-, green-, and red-fluorescent dots or stripes is provided. Inside the panel, a shadow mask in which a large number of electron beam passing holes have been made is provided so as to face the fluorescent screen. In the neck, an electron gun assembly assembly for generating three electron beams is provided. The electron beams are deflected horizontally and vertically by horizontal and vertical deflection fields generated by the deflecting yoke and directed to the fluorescent screen via the shadow mask. Then, the electron beams scan the fluorescent screen horizontally and vertically, thereby causing a color image to appear on the screen.
One color picture tube of such a type is a self convergence in-line color tube, which has been widely used. In the self convergence in-line color tube, the in-line electron gun assembly produces in-line three electron beams passing in the same horizontal plane. The three electron beams in a line emitted from the electron gun assembly are deflected by a pincushion horizontal deflection magnetic field and a barrel vertical deflection magnetic field both generated by the deflection yoke, thereby causing the three electron beams in a line to converge together all over the screen without using special correction means.
In such a cathode-ray tube, the deflecting yoke is a heavily power consuming source. To reduce the power consumption of the cathode-ray tube apparatus, it is important to reduce the power consumption of the deflecting yoke. To increase the screen luminance, the cathode voltage to accelerate the electron beams must eventually be raised. In addition, to meet the requirements for OA devices, such as HD (High Definition) TVs or PCs (Personal Computers), the deflection frequency has to be increased. Both the raised cathode voltage and the increased deflection frequency result in an increase in the deflection power.
In the case of OA devices, such as PCs, which the operator uses sitting close at the cathode-ray tube, magnetic fields leaking from the deflecting yoke to the outside of the cathode-ray tube, or leakage magnetic fields, have been regulated strictly. A widely used means of reducing the magnetic field leaking from the deflecting yoke to the outside of the cathode-ray tube is to add a compensating coil. The addition of the compensating coil, however, results in an increase in the power consumption.
To reduce the deflection power or the leakage magnetic field, the diameter of the neck of a cathode-ray tube should be made smaller and therefore the outside diameter of the yoke section on which the deflecting yoke is installed is made smaller, thereby making the acting space of the deflecting magnetic field smaller, which allows the deflecting magnetic field to act on the electron beams efficiently.
In a conventional cathode-ray tube, since the electron beams pass near the inside face of the yoke section installed on the deflecting yoke, the still smaller neck diameter and yoke outside diameter permit the electron beams advancing toward the diagonal section of the fluorescent screen at the maximum deflection angle to impinge on the inner wall of the yoke section, which allows a portion where no electron beam impinges to appear on the fluorescent screen. As a result, with the conventional cathode-ray tube, it is difficult to reduce the deflection power by making the neck diameter and yoke section outside diameter smaller. If electron beams continue striking the inner wall of the yoke section, the temperature at the impinged portion will rise so high that the glass will melt, leading to the danger of implosion.
A means for solving such a problem has been disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 48-34349 (U.S. Pat. No. 3,731,129). In the publication, the yoke section 110 of the funnel 103 on which a deflecting yoke is installed is shaped so that its form changes gradually from round at the neck 104 to almost rectangular at the panel 102, as shown in FIGS. 1B to 1F, which are sectional views taken along line Bxe2x80x94B to Fxe2x80x94F, respectively, in an cathode-ray tube 113 of FIG. 1A. The shape is based on the idea that, when a rectangular raster is drawn on the fluorescent screen, the area through which the electron beams pass should take the form of an almost rectangular shape.
The formation of the yoke section 110 on which the deflecting yoke is installed into a pyramid shortens the major axis (horizontal axis: H-axis) and minor axis (vertical axis: V-axis) of the deflecting yoke. Therefore, bringing the horizontal and vertical deflection coils of the deflecting yoke close to the electron beams enables the beams to deflect efficiently, which helps reduce the deflection power. With such a cathode-ray tube, as the yoke section is made more rectangular to reduce the deflection power effectively, the strength of the vacuum envelope to atmospheric pressure decreases due to the distortion of the glass, impairing safety.
Recently, there have been strong demands toward external light reflection and easy-to-see pictures. Thus, it is indispensable to make the panel flatter. Since the flatter panel surface of the cathode-ray tube decreases the vacuum strength, direct use of the conventional funnel with the pyramidal yoke section would fail to secure the bulb strength necessary for safety.
For this reason, there has been a problem of being unable to make the cross section of the yoke section rectangular enough to reduce the deflection power sufficiently. Another problem is that the strength of the bulb to atmospheric pressure is so low that it cannot be applied to a flat panel.
In connection with the technique for shaping the yoke section into a pyramid, in about 1970, the applicant mass-produced two series of cathode-ray tubes: one with a deflection angle of 110xc2x0, a neck diameter of 36.5 mm, and panel diagonal diameters of 18xe2x80x3, 20xe2x80x3, 22xe2x80x3, and 26xe2x80x3, and the other with a deflection angle of 110xc2x0, a neck diameter of 29.1 mm, and panel diagonal diameters of 16xe2x80x3 and 20xe2x80x3. At that time, the technique was applied to a cathode-ray tube called a 1R tube (hereinafter, referred to as a 1R square yoke section tube), the outer surface of whose panel was almost spherical, the curvature radius of the panel outer surface being about 1.7 times the screen diagonal effective diameter. As for cathode-ray tubes the curvature radius of whose panel outer surface was twice or more the screen diagonal effective diameter, the relationship between the shape of the yoke section and the strength of the bulb was unknown.
As described above, recently, there have been demands toward reducing the deflection power and leakage magnetic field in a cathode-ray tube apparatus. It is very difficult to meet the demands while achieving higher luminance and higher frequency required for OA devices, including HD TVs and PCs. A conventional structure to reduce the deflection power is such that a pyramidal yoke section changing from round at the neck to almost rectangular at the panel is formed on the yoke section on which a deflecting yoke is to be installed. Although such a structure has been proposed, it is difficult to produce a vacuum envelope that not only ensures a sufficient strength to atmospheric pressure but also reduces the deflection power sufficiently.
The object of the present invention is to provide a cathode-ray tube apparatus which not only secures a sufficient strength of the vacuum envelope to atmospheric pressure even when the yoke section is made pyramidal but also reduces the deflection power effectively, thereby meeting the demands for higher luminance and higher-frequency deflection.
The present invention has been centered on the funnel of the enclosure of a cathode-ray tube, particularly on the shape of the yoke section. The funnel section is part of the vacuum envelope between the panel section having a fluorescent screen on its inside face and the neck section having an electron gun assembly in it and connects the panel section to the neck section. The funnel section is composed of a panel-side increased-diameter funnel section (a first section), and a neck-side decreased-diameter, almost pyramidal yoke section (a second section).
In the present invention, with at least one cross section perpendicular to the tube axis of the yoke section being made noncircular and allowed to have an outside diameter of the yoke section that becomes the largest between the directions of vertical axis and horizontal axis of the screen, if the outside diameter of the yoke section in the vertical direction is SA, the outside diameter of the yoke section in the horizontal direction is LA, and the maximum outside diameter of the yoke section is DA, an index value a indicating the degree of rectangle for the noncircular shape is defined as
xcex1=(SA+LA)/(2*DA).
Under these conditions, the yoke section is so formed that it meets the following expression:
0.00xe2x89xa6(xcex10 xe2x88x92xcex1min)xe2x89xa60.04
where xcex10 is the index value at the deflection reference position and xcex1min is the minimum of the index values in the whole area of the yoke section.
Furthermore, the yoke section is so formed that it meets the following expression with respect to the index value xcex10 indicating the degree of rectangle at the deflection reference position:
xe2x88x920.04xe2x89xa6(xcex10 xe2x88x92xcex1s)xe2x89xa60.04
wherein xcex1s is the index value of the degree of rectangle at a position between the deflection reference position on the yoke section and the screen-side end of the yoke section.